Numerous methods for producing hydrogen gas are known in the art. The production of industrial-scale volumes of hydrogen is typically accomplished by application of the steam-methane reforming process, which entails the catalytic reforming of natural gas with steam at elevated temperatures (800-950° C.). This process yields a crude synthesis gas, which is a mixture of hydrogen, carbon monoxide, and carbon dioxide, and the crude synthesis gas is further reacted in a catalytic water-gas shift conversion step to convert carbon monoxide with water to additional hydrogen and carbon dioxide. The shifted synthesis gas is purified to yield a final hydrogen product containing greater than 99 volume % hydrogen.
An alternative process for the production of hydrogen is the partial oxidation of methane to form synthesis gas, which is subsequently shifted if necessary and purified. Partial oxidation is known to be highly exothermic.
Another alternative process to generate synthesis gas for hydrogen production is autothermal reforming, which is essentially a thermally balanced combination of the steam-methane reforming process and partial oxidation. One considerable drawback associated with these alternative processes is that partial oxidation requires a supply of high purity oxygen gas to the reaction system. Therefore, the use of these processes requires the additional step of separating air to produce the oxygen gas, and the air separation process increases the capital and operating costs of hydrogen production.
More recently, a new process for producing hydrogen utilizing complex metal oxides has been disclosed in U.S. patent application Ser. No. 11/165,720 (Pub. No. U.S. 2006-0292066-A1), Ser. No. 11/165,731 (Pub. No. US 2006-0292069-A1), Ser. No. 11/339,060 (Pub. No. 2007-0172418-A1), Ser. No. 11/339,806 (Pub. No. US 2007-0172419-A1), and Ser. No. 11/737,942.
The hydrogen producing process using complex metal oxides may be operated in an autothermal and cyclic manner for the synthesis of hydrogen from a hydrocarbon, (e.g. methane), steam and air. It utilizes the complex metal oxide as a CO2 sorbent and as a source of oxygen for methane oxidation during reforming of methane with steam. In the production step of the process, largely pure hydrogen is obtained by the reaction of steam and methane in the presence of the complex metal oxide and a steam hydrocarbon reforming catalyst. The methane oxidation during the production step reaction provides heat for the reforming reaction. During the production step, the complex metal oxide sorbs CO2 and the complex metal oxide is reduced. In the regeneration step of the process, the spent complex metal oxide is regenerated with air or other oxygen-containing gas, thereby liberating CO2, and reconstituting the complex metal oxide.
Complex metal oxide is generally synthesized in a powder form. Since the use of powder in a fixed bed reactor may result in an unacceptably high pressure drop through the reactor, it would be desirable to form this powder into pellets for use in a fixed bed reactor. Pellets are useful to allow a reasonable pressure drop through the fixed bed reactor. Pellets may also be useful for ebullated, expanded, or fluidized beds.
It has been found that conventional methods for forming pellets are not suitable for forming dimensionally stable pellets from complex metal oxides for long term use in hydrogen production. Complex metal oxide material undergoes more than a 40% volumetric expansion and contraction due to the sorption and desorption of CO2. Because of the expansion and contraction, complex metal oxide-containing pellets prepared using conventional binders and conventional techniques such as extrusion or pelletization of powder fall apart within a few CO2 sorption and desorption cycles. Furthermore, complex metal oxide-containing pellets using conventional binders and conventional techniques do not have sufficient crush strength to be useful in a fixed bed reactor.
It would be desirable to produce hydrogen in a fixed bed reactor using complex metal oxide-containing pellets having structural and dimensional stability.
It would be desirable to form complex metal oxide-containing pellets having structural and dimensional stability from complex metal oxide powder.